BIO L2

• Phylum Porifera:


  Sponges are sessile with porous bodies and choanocytes
• Based on both molecular evidence and the morphology on their choanocytes, sponges represent the lineage closest to the colonial choanoflagellates.
• Sponges are sessile animals that lack nerves or muscles. However, individual cells can sense and react to changes in the environment
• The 9,000 or so species of sponges range in height from about 1 cm to 2 m and most are marine. About 100 species live in fresh water.
• The germ layers of sponges are loose federations of cells, which are not really tissues because the cells are relatively unspecialized.
• The body of a simple sponge resembles a sac perforated with holes. Water is drawn through the pores into a central cavity, the spongocoel, and flows out through a larger opening, the osculum.
• Nearly all sponges are suspension feeders, collecting food particles from water passing through food-trapping equipment.
• Flagellated choanocytes, or collar cells, line the spongocoel (internal water chambers) create a flow of water through the sponge with their flagella, and trap food with their collars.
• The body of a sponge consists of two cell layers separated by a gelatinous region, the mesohyl. Wandering through the mesohyl are amoebocytes. They take up food from water and choanocytes, digest it, and carry nutrients to other cells.
• sponges also secrete tough skeletal fibres within the mesohyl. In some groups of sponges, these fibres are sharp spicules of calcium carbonate or silica. Other sponges produce more flexible fibres from a collagen protein called spongin. We use these pliant, honeycombed skeletons as bath sponges.
• Most sponges are hermaphrodites, with each individual producing both sperm and eggs. Gametes arise from choanocytes or amoebocytes. The eggs are retained, but sperm are carried out the osculum by the water current. Sperm are drawn into neighbouring individuals and fertilize eggs in the mesohyl.
• The sponge zygotes develop into flagellated, swimming larvae that disperse from the parent. When a larva finds a suitable substratum, it develops into a sessile adult
• Sponges are capable of extensive regeneration, the replacement of lost parts. They use regeneration not only for repair but also to reproduce asexually from fragments broken off a parent sponge.
• Phylum Cnidaria:


  Cnidarians have radial symmetry, a gastrovascular cavity, and cnidocytes
• The cnidarian basic body plan has two variations:

  the sessile polyp and the floating medusa
• The cylindrical polyps, such as hydras and sea anemones, adhere to the substratum by the aboral end and extend their tentacles, waiting for prey. Medusas (also called jellies) are flattened, mouth-down versions of polyps that move by drifting passively and by contacting their bell-shaped bodies
• Some cnidarians exist only as polyps. Others exist only as medusas. Still others pass sequentially through both a medusa stage and a polyp stage in their life cycle
• Cnidarians are carnivores that use tentacles arranged in a ring around the mouth to capture prey and push the food into the gastrovascular chamber for digestion. Batteries of cnidocytes on the tentacles defend the animal or capture prey. Organelles called cnidae evert a thread that can inject poison into the prey, or stick to or entangle the target. Cnidae called nematocysts are stinging capsules.
• The phylum Cnidaria is divided into four major classes:

  Hydrozoa, Scyphozoa, Cubozoa and Anthozoa.
• Most hydrozoans alternate polyp and medusa forms, as in the life cycle of Obelia. The polyp stage, often a colony of interconnected polyps, is more conspicuous than the medusas.
• Hydras (freshwater) exist only in the polyp form. When environmental conditions are favourable, a hydra reproduces asexually by budding, the formation of outgrowths that pinch off from the parent to live independently. When environmental conditions deteriorate, hydras form resistant zygotes that remain dormant until conditions improve.
• In Scyphozoa, the medusa generally prevails in the life cycle. The medusas of most species live among the plankton as jellies. Most coastal scyphozoans go through small polyp stages during their life cycle. Jellies that live in the open ocean generally lack the sessile polyp.
• Anthozoa (Sea anemones and corals) occur only as polyps. Coral animals live in solitary or colonial forms and secrete a hard external skeleton of calcium carbonate. Each polyp generation builds on the skeletal remains of earlier generations to form skeletons that we call coral. In tropical seas, coral reefs provide habitat for a great diversity of invertebrates and fishes.
• Cubozoa (Box jellyfish) life cycle includes a medusa and small polyp stage. Have a small transparent cube-shaped medusa form with at least four tentacles. More developed than Schyphozoa, more rapid and direct movement, more advanced nervous system and functioning eyes. Are among the most dangerous marine animals e.g. sea wasps.
• Flatworms are acoelomates with gastrovascular cavities. There are about 20,000 species of flatworms living in marine, freshwater, and damp terrestrial habitats. They also include parasitic species, such as the flukes and tapeworms. Flatworms have thin bodies, ranging in size from nearly microscopic to tapeworms over 20 m long. Flatworms are triploblastic, with a middle embryonic tissue layer, mesoderm, which contributes to more complex organs and organ systems and to true muscle tissue.
• While flatworms are structurally more complex than cnidarians, they are simpler than other bilaterans. Like cnidarians and ctenophores, flatworms have a gastrovascular cavity with only one opening (and tapeworms lack a digestive system entirely and absorb nutrients across their body surface). Unlike other bilaterians, flatworms lack a coelom.
• Flatworms are divided into four classes:

  Turbellaria, Monogenia, Trematoda, and Cestoidea.